Response Control Systems are constructed such that, with the help of special devices, an as large as possible part of the earthquake's energy is prevented from inducing stresses as potential energy in the system. By the layout of the behavior of these devices, their position in the system and the structure of the remaining system tuned accordingly, the whole system becomes a mechanism reacting in a controlled manner to a strong motion earthquake and therefore is not a "static" system in its pure sense anymore.
This mechanism can be designed such that it limits the stresses in the system to a defined level. This safely protects the structure from damage. Such a limit can be achieved by implementing friction dampers that do not transfer more than the adjusted friction force to the structural system.
There is a distinction between passive, active and hybrid Response Control Systems.
To function, active systems need external energy. For earthquake protection of buildings, active systems are not suitable in most cases because the required machinery and energy approaches magnitudes that are often not realistic. They have been implemented successfully though, in high-rise buildings to control wind induced vibrations.
On the other hand, passive systems have proven their effectiveness for earthquake protection and have been implemented successfully several times in different countries, notably in Japan. Especially worth noting are several Base Isolation Systems, although they are limited to buildings of no more than approximately 10 stories. Furthermore, the Hyde Systems should be noted which have been successfully implemented in high-rises with more than 30 stories and also in low-rise buildings. These are systems where hysteretic devices are exclusively used for response control.
So far, hybrid systems have no practical application but they are promising with respect to improving the effect of passive systems in case of an earthquake.
Dampers play an important role in the above mentioned Response Control Systems, may they be active, passive or hybrid. Dampers can be distinguished by their effective mechanism like friction, plasticity, viscosity, magnetism or mass inertia. Depending on their design, they allow unidirectional, bi-directional or tri-directional action. Important criteria for a damper are accuracy and stability of its hysteresis under limit load, service life, maintainability, adaptability to various applications and implementation requirements.
For retrofitting buildings that are vulnerable to earthquakes, Base Isolation and Hyde Systems are particularly useful. Base Isolation is suited for protection of historic buildings where the structural material and construction cannot be altered because of its historic importance. Furthermore, the Base Isolation System is essentially invisible underneath the buildings after completion.
Hyde Systems are suited for retrofitting of many steel and reinforced concrete structures built in the sixties and seventies that are vulnerable to earthquakes because of then insufficient code requirements.
Friction dampers that can be used in such systems are for example known from the European patent application no. 0349979. The friction damper described there is a friction-spring element that is made of outer rings and axially slotted inner rings acting together via conical contact surfaces. The rings are arranged inside a casing which consists of two telescoping parts. Each of these parts has a spherical rod end bearing attached to its free end. The prestressing of the friction-spring element is accomplished by a central bolt within the casing.
This friction-spring element is only effective in one direction and has an irregular transition from sticking to sliding. Furthermore, the adjustment of the prestressing is inaccurate and the adjustment bolt is difficult to reach.
From the German patent no. 3402449, a device to dampen vibrations in tower-like structures is known. It consists of a pendulum which is suspended with one end from a cantilever attached to the building and loosely enters with the other end through an opening at the top of a void in a friction weight. The friction weight comprises a stack of several un-bonded circular shaped disc-like friction plates with outer diameters increasing from top plate to bottom plate. The dimensioning of the friction plates should be performed in such a way that the damping effect of the stacked friction plates increases with respect to a particular pendulum displacement and the layout of the device is simplified. This allows the damping device to react already at small building motions since with growing pendulum amplitude, more and more friction plates are activated.
Another damper is known from the German patent no. 3414706 which acts like a tuned mass damper with a gradually increasing damping effect after the building reaches a certain amplitude of vibration. For this purpose, the damper consists of one or several friction objects with convex friction surfaces. Each friction object is fixed to the top of an essentially vertical elastic cantilever column which is attached to the building. One friction disc with concave friction surfaces is located at a certain distance over each friction object in the at-rest condition and joined with one damper mass. Each damper mass is attached to a separate pendulum suspended from a cantilever which is fixed to the building. Each friction disc has a circular asperity centered over the at-rest position of the friction object belonging thereto. When the pendulum exceeds a certain amplitude, the friction object slides along the concave friction surface of the friction disc and its circular asperity. Because of this layout, a friction force does not occur around the center position of the damping device and the damper mass can respond already to small vibrations.
From the two documents mentioned above, friction dampers with friction discs or the use of convex friction discs are known as single features.
Considering this state of the art, it is an object of this invention to provide a friction damper that is able to dampen vibrations in two dimensions, can be re-used, is easily adjustable in a wide range even when implemented and has a very smooth transition from sticking to sliding in particular to prevent the stick-slip motion present in known friction dampers.
Furthermore, it is an object of this invention to provide a maintenance-friendly damper with a very high adjustment accuracy.